The present invention relates to a mounting device for a diffraction grating used for a rapid scanning monochromator, and more particularly to a mounting device for a diffraction grating in which the scanning velocity is equal over the entire wavelength of the monochromator.
In the study of various spectra, the importance of rapid spectra measuring over a certain wavelength region has recently been noted, and a new method has been proposed in place of the conventional photographic spectra measuring. In the new proposal, a semiconductor light detecting element which is a multi-channel detector is placed at an exit focal plane and the wavelength scanning is carried out according to a completely electrical method. However, the proposed new method is applicable only to monochromators and spectrographs which have a flat field of focus. Also, it uses expensive elements as for example in the scanning circuits. The sensitivity of the light receiving element itself and spectroscopic sensitivity characteristic are limited to a certain extent.
On the other hand, a photomultiplier has high sensitivity, is not expensive and can provide various spectroscopic sensitivity characteristics. Therefore it is suitable for use as a light detecting element for the rapid scanning monochromator.
Such a rapid scanning monochromator provided with an oscillating folding mirror placed in the very front of an exit slit is known, in which the light path is changed by the oscillation of the mirror and light in a certain wavelength region is passed through the slit repeatedly to measure the spectra. According to this method, however, by the change of angle of the oscillating mirror the opening angle of a light collecting mirror placed at the exit side changes and as a result, the spectral intensity is not constant. Also, a linear relation does not exist between the rotational angle of the oscillating mirror and the irradiated wavelength, thus requiring further complicated and very expensive electrical converters for correctly indicating and recording the spectra.
A further monochromator of the photomultiplier type which uses a combination of a cam and a bar attached to a grating axle or a combination of a screw and a sine bar is known. In the former, a bar or an arm mounted on the rotating axle of the diffraction grating is driven by a cam. In the latter case, a sine bar coaxially fixed to the diffraction grating is attached to a roller mounted around a screw. By the rotation of the screw, the roller moves, and therefore linear wavelength scanning is carried out with respect to the rotational angle of the driving axle of the screw. In the former case, the cam is continuously rotated with certain velocity and thereby the diffraction grating is oscillated so that a rapid scanning is carried out. In this method, the above-mentioned problem of the change of the opening angle of the light collecting mirror and the linearity of the wavelength may be solved, but it is difficult to manufacture a cam of the wavelength linear type. Also it is not possible to obtain linear scanning over certain wavelength region of the monochromator with the same cam. In the latter method using a sine bar and a screw, the screw repeats a recriprocating rotation and therefore it may not be used for a ultra-high speed reciprocating movement.